1. Field of the Invention
The present invention relates to video systems. More specifically, the present invention relates to methods and circuits for displaying multiple video signals on a single display.
2. Discussion of Related Art
Analog video displays such as cathode ray tubes (CRTs) dominate the video display market. Thus, most electronic devices that require video displays, such as computers and digital video disk players, output analog video signals. As is well known in the art, an analog video display sequentially reproduces a large number of still images to give the illusion of full motion video. Each still image is known as a frame. For NTSC television, 30 frames are displayed in one second. For computer applications, the number of frames per seconds is variable with typical values ranging from 56 to 100 frames per seconds.
FIG. 1(a) illustrates a typical analog video display 100. Analog video display 100 comprises a raster scan unit 110 and a screen 120. Raster scan unit 110 generates an electron beam 111 in accordance with an analog video signal VS, and directs electron beam 111 against screen 120 in the form of sequentially-produced horizontal scanlines 101–109, which collectively form one frame. Screen 120 is provided with a phosphorescent material that is illuminated in accordance with the video signal VS transmitted in electron beam 111 to produce contrasting bright and dark regions that create an image, such as the diamond shape shown in FIG. 1(a). After drawing each scanline 101–108, raster scan unit 110 performs a horizontal flyback 130 to the left side of screen 120 before beginning a subsequent scanline. Similarly, after drawing the last scanline 109 of each frame, raster scan unit 110 performs a vertical flyback 131 to the top left corner of screen 120 before beginning a subsequent frame. To avoid generating an unwanted flyback traces (lines) on screen 120 during horizontal flyback 130, video signal 130 includes a horizontal blanking pulse that turn off electron beam 111 during horizontal flyback 130. Similarly, during vertical flyback 135, video signal VS includes a vertical blanking pulse that turns off electron beam 111 during vertical flyback 135.
FIG. 1(b) illustrates a typical analog video signal VS for analog video display 100. Video signal VS is accompanied by a horizontal synchronization signal HSYNC and a vertical synchronization signal VSYNC (not shown). Vertical synchronization signal VSYNC contains vertical sync marks to indicate the beginning of each new frame. Typically, vertical synchronization signal VSYNC is logic high and each vertical sync mark is a logic low pulse. Horizontal synchronization signal HSYNC contains horizontal sync marks (logic low pulses) 133, 134, and 135 to indicate the beginning of data for a new scanline. Specifically, horizontal sync mark 133 indicates video signal VS contains data for scanline 103; horizontal sync mark 134 indicates video signal VS now contains data for scanline 104; and horizontal sync mark 135 indicates video signal VS now contains data for scanline 105.
Video signal VS comprises data portions 112, 113, 114, and 115 that correspond to scanlines 102, 103, 104, and 105, respectively. Video signal VS also comprises horizontal blanking pulses 123, 124 and 125, each of which is located between two data portions. As explained above, horizontal blanking pulses 123, 124, and 125 prevent the electron beam from drawing unwanted flyback traces on analog video display 100. Each horizontal blanking pulse comprises a front porch FP, which precedes a horizontal sync mark, and a back porch BP, which follows the horizontal sync mark. Thus, the actual video data for each row in video signal VS lies between the back porch of a first horizontal blanking pulse and the front porch of the next horizontal blanking pulse. In color video signals, color data is multiplexed with luminance information in the data portions of video signal VS.
Typically, video signal VS contains a luminance signal and two chrominance signals. The luminance signal, generally referred to as Y, corresponds to the brightness information for the image. The two chrominance signals, generally referred to as U and V, provide the color information. Multiplexed analog video signals are generally referred to as YUV format. However, some video signals, such as VGA, SVGA, XGA used in the computer industry use a red signal, a green signal and a blue signal. The individual color signals are combined into a composite video signal in RGB format.
In general, video displays used in the computer industry have much higher resolution and refresh rates than video display units used in the entertainment industry. Thus, most video display units for computers are incompatible with video signals used in for television or other entertainment industry devices such as DVD players and video-cassette recorders (VCRs). Similarly, most televisions are incompatible with computers. However, as the size of video display units used by computers has increased, computer users have a desire to use the video display units to display multiple video signals to reduce the cost and space required for having a separate video display unit for different type of video signals. Furthermore, many users have a desire to be able to watch multiple video signals simultaneously. For example, FIG. 2 illustrates a video display unit 210 being used to simultaneously display a first analog video signal AVS1 and a second analog video signal AVS2. Specifically, video display unit 210 includes a display screen 212, which is used to display images from first analog video signal AVS1, which could be generated by a computer for example. Superimposed within the images on display screen 110 is a picture-in-picture (PIP) window 120. PIP window 120 includes images from second analog video signal AVS2, which could be generated by a DVD player for example. In general, PIP window 120 is used for the lower resolution video signal.
As explained above, video display unit 210 is generally compatible with only a limited number of video signal formats. Thus to use video display unit 210 for both first analog video signal AVS1 and second analog video signal AVS2, a video system 220 is used to combine first analog video signal AVS1 and second analog video signal AVS2 into a combined analog video signal CAVS, which has a format compatible with video display unit 210.
FIG. 3, shows a simplified block diagram of a conventional video system 300, which can be used to combine first analog video signal AVS1 and second analog video signal AVS2 to form combined video signal CAVS. Video system 300 includes a analog to digital converter 310, a video scaler 320, an analog to digital converter 330, a digital frame buffer 340 and a digital to analog converter 350. Analog to digital converter 330 converts first analog video signal AVS1 into a first digital video signal DVS1. In digital form, each image within digital video signal DVS1 is a two-dimensional array of pixels which correspond to display screen 212 (FIG. 2). These pixels are stored in digital frame buffer 340, except for the pixels which would correspond with PIP window 214 (FIG. 2).
Analog to digital converter 310 converts second analog video signal AVS2 into a second digital video signal DVS2. Video scaler 320 processes second digital video signal DVS2 to produce scaled digital video signal SDVS2. Video scaler 320 may also change the format of the video signal, for example for YUV format to RGB format. The images in digital video signal DVS2 are also a two-dimensional array of pixels and are scaled to fit in PIP WINDOW 214. The pixels forming the images of scaled digital video signal DVS2 are stored digital frame buffer 340 at locations corresponding with PIP window 214. The pixels in digital frame buffer 340 are combined to form a combined digital video signal CDVS, which includes the images originally from analog video signal AVS2 superimposed over the images originally from analog video signal AVS1. Digital to analog converter 350 converts combined digital video signal CDVS into a combined analog video signal CAVS, which can be used with a video display unit, such as video display unit 210 (FIG. 2).
By processing the video signals in digital form, differing refresh rates and scan rates are easily remedied using digital frame buffer 340. Specifically, digital frame buffer 340 is made to have two independent write ports and one independent read port. Thus, pixels originating from analog video signal AVS1 can be written into digital frame buffer 340 at different rate than pixels originating from analog video signal AVS2. Furthermore, pixels can be read out of digital frame buffer 340 at yet another different rate if desired. However, for high resolution graphics digital frame buffer 340 must be very large and very fast and thus very expensive. The cost of the digital frame buffer 340 is further increased by the need for three independent ports. Hence, there is a need for a system or method to combine analog video signals without requiring an expensive digital frame buffer.